Measure how many tasks are cancelled in MessageLoop.

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 74287b148e61f7d5165968393e41f3ac2e2b081f..55242db0a978bf79268bfbdf7fb1af9b3fd9d782 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -13,6 +13,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_pump_default.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/run_loop.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/thread_id_name_manager.h"
@@ -537,6 +538,8 @@ bool MessageLoop::DoDelayedWork(TimeTicks* next_delayed_work_time) {
   if (next_run_time > recent_time_) {
     recent_time_ = TimeTicks::Now();  // Get a better view of Now();
     if (next_run_time > recent_time_) {
+      UMA_HISTOGRAM_BOOLEAN("TaskRunner.WokeUpTooSoonTaskIsCancelled",
+                            delayed_work_queue_.top().task.IsCancelled());
       *next_delayed_work_time = next_run_time;
       return false;
     }
@@ -546,8 +549,14 @@ bool MessageLoop::DoDelayedWork(TimeTicks* next_delayed_work_time) {
       std::move(const_cast<PendingTask&>(delayed_work_queue_.top()));
   delayed_work_queue_.pop();
 
-  if (!delayed_work_queue_.empty())
+  UMA_HISTOGRAM_BOOLEAN("TaskRunner.ExecutingDelayedTaskIsCancelled",
+                        pending_task.task.IsCancelled());
+
+  if (!delayed_work_queue_.empty()) {
+    UMA_HISTOGRAM_BOOLEAN("TaskRunner.ScheduledTaskIsCancelled",
+                          delayed_work_queue_.top().task.IsCancelled());
     *next_delayed_work_time = delayed_work_queue_.top().delayed_run_time;
+  }
 
   return DeferOrRunPendingTask(std::move(pending_task));
 }